Oxygen sensor

ABSTRACT

The present invention relates to an oxygen sensor, in which in order to be strong against vibration and shocks as well as to make rapid heating possible and to be capable of rapidly performing correct detection, the oxygen sensor provided is composed of a sensor element having a body portion and a thin portion which is thinner than the body portion, a detecting element is formed at the thin portion for measuring oxygen concentration, and an exothermic element is provided for heating the detecting element.

TECHNICAL FIELD

The present invention relates to an oxygen sensor which is used fordetecting oxygen concentration in an exhaust gas from an internalcombustion engine and the like.

BACKGROUND ART

For detecting oxygen concentration in an exhaust gas from an internalcombustion engine, in order to exhibit a detecting function, an oxygensensor is employed composed of a sensor element formed with anexothermic element for heating an oxygen concentration detecting portionbeing integrally laminated with the detecting portion.

Such an oxygen sensor is disclosed in, for example, the Japaneseexamined Patent Publication No. 36461-1988 or the Japanese Laid-openPatent Application No. 222159-1987.

As shown in FIG. 11, the former publication discloses electrodes 105 and107 which are laminated on both faces of a solid electrolyte layer 106,which are laminated on an insulating body 101 having a gas penetratingproperty to form an oxygen concentration detecting portion, on which islaminated an exothermic element 103 to form a sensor element 1 of a thinplate configuration. On the other hand, as shown in FIG. 12, the latterapplication discloses a side face of a cylinder-shaped insulatingmaterial 110 having an air introducing hole 110a provided and a throughhole 110b communicates with the air introducing hole 110a. An oxygenconcentration detecting layer in which electrodes 105 and 107 arelaminated on both faces of a solid electrolyte layer 106, is laminatedon an exothermic element 103. Element 103 is then laminated on theinsulating material 110 covering the through hole 110b to form a hollowand cylinder-shaped sensor element 1.

However, in the Japanese Patent Publication No. 36461-1988 the sensorelement has a thin plate configuration, causing problems since it isweak from a view of strength to easily suffer breakage and damage due tovibration and shocks. On the other hand, Japanese Patent ApplicationLaid-open No. 222159-1987 has a large heat capacity on account of thehollow and cylinder-shaped sensor element which has a low heatingefficiency of the sensor element by the exothermic element, so thatthere is a problem, since it cannot exhibit its function immediatelyduring the start-up of an internal combustion engine and the like inwhich rapid heating of the sensor element is required.

The present invention has taken the above circumstances intoconsideration, the object of which is to provide an oxygen sensor whichis composed of a sensor element being strong against vibration andshocks and capable of performing rapid heating.

DISCLOSURE OF INVENTION

In order to achieve the above mentioned object, the present inventionprovides an oxygen sensor comprising a sensor element having a bodyportion and a thin portion which is thinner than the body portion, adetecting element formed at the thin portion for measuring oxygenconcentration, and an exothermic element formed at the thin portion ofthe sensor element for heating the detecting element.

According to the above mentioned means, the sensor element has the thinportion formed to be thinner than the body portion, so that the weightof the forward end portion of the sensor element decreases, thereby thestress due to the vibration and shocks applied to a supporting andfixing portion of the sensor element decreases, and the detectingelement is formed at the thin portion, thereby the heating efficiency ofthe sensor element by the exothermic element increases.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an exploded perspective view showing an example of a sensorelement in an oxygen sensor of the present invention,

FIG. 2 is a partial cross-sectional view of the oxygen sensor providedwith the above mentioned sensor element,

FIG. 3 is a graph showing a test result of temperature-increasingcharacteristics of each oxygen sensor,

FIG. 4 is a perspective view of a sensor element,

FIG. 5(a), (b), and (c) are illustrative views showing another example,

FIG. 6 and FIG. 7 are exploded perspective views showing other examplesof sensor elements,

FIG. 8(a) is an illustrative view showing another example,

FIG. 8(b) is a cross-sectional view of FIG. 8(a),

FIG. 9 and FIG. 10 are illustrative views showing other examples ofsensor elements, and

FIG. 11 and FIG. 12 are exploded perspective views of sensor elements inconventional oxygen sensors.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be explained hereinafter according toexamples shown in the figures. FIG. 1 is a development view showing anexample of a sensor element in an oxygen sensor of the presentinvention.

In FIG. 1, 101 is a square pillar-shaped support having a through holewith both ends opened comprising alumina ceramics, zirconia ceramics orthe like, and an opening end face 101a of the support 101 at the sidesubjected to disclosure to be exposed to an exhaust gas and makes aslant face which is continuously thinned from one face of side faces toform a thin portion 101b. When viewed in the side direction, the forwardend portion of support 101 has a wedge-shaped configuration, and theother end has an air introducing hole 111 for introducing air via a bodyportion 1b. An exothermic element 103 mainly comprising a platinum metalmaterial, is subjected to lamination at the face opposing the slant faceof the above mentioned support 101 via an insulating layer 102 such asalumina or the like so as to completely cover the above mentionedopening portion at the slant face 101a by an exothermic portion of theexothermic element 103 as viewed in a projection drawing. Exothermicelement 103, excluding terminal electrode portions 114a and 114b islaminated an insulating layer 104 comprising the same material as thatof the insulating layer 102. A solid electrolyte layer 106 composed of,for example, zirconia to which yttrium is added, which has a referenceelectrode 105 and a measuring electrode 107 being laminated so as tocoincide the opening portion at the slant face 101a of the support 101at the side of the exhaust gas with the electrode 105 to form a sensorportion which is a detecting element 1a. The reference electrode 105 isconnected with a reference electrode terminal 112b via a through hole113 provided at the solid electrolyte 106, and the measuring electrode107 is connected with a measuring electrode terminal 112a, respectively.

A protecting layer 108 is provided for preventing the sensing electrode107 from direct exposure to a gas to be measured, and is a protectinglayer comprising an inorganic material such as porous alumina, spinel orthe like.

Hence, the support 101 is formed by injection molding or the like, andthe solid electrolyte layer 106 is formed by extrusion molding or thelike, and the electrode layers 105 and 107, the exothermic element 103,and the insulating layers 102 and 104 are provided by a printing methodor the like, which are laminated in a state of no sintering, after whichsimultaneous sinter is carried out to form the sensor element.

Incidentally, the protecting layer 108 is formed by flame spray coatingof powder of a raw material after sintering of the above mentionedsensor element. FIG. 2 is a partial cross-sectional view of an oxygensensor into which the sensor element formed by the above mentionedmethod is incorporated.

In FIG. 2, 1 is the sensor element, and 2 is a housing having aprotecting cover 3 for preventing the sensor portion 1a of the sensorelement 1 from direct exposure to the exhaust gas, in which the sensorelement 1 is incorporated into the housing 2 to be fixed by means of amethod of thermal caulking or the like via a powder material 4 such astalc or the like. Further, a lead wire 5 is connected with the terminalelectrode portion of the sensor element 1 by means of brazing or thelike, and the lead wire 5 is pulled out to the outside of the oxygensensor body.

Next, with respect to the above mentioned example, its function will beexplained.

In the present invention, the sensor element of the oxygen sensor hasthe portion subjected to disclosure to be exposed to the exhaust gasprovided with the thinned portion subjected to thinning in accordancewith directing toward the forward end as described above, so that theheat capacity of the forward end portion which is heated by theexothermic element 103 contained in the sensor element 1 becomes small,therefore, the heating efficiency by the exothermic element 103increases, as result, temperature rises rapid.

FIG. 3 relates to the oxygen sensor of the present invention and aconventional oxygen sensor, which is the result of a test oftemperature-increasing characteristics of the forward end portions ofthe sensor elements in the case of heating by the exothermic elementshaving the same specification contained in each sensor element.

However, with respect to the shape and size of the sensor element, forthe conventional one was used a square pillar-shaped element having awidth of 5 mm, a thickness of 5 mm, and a length of 60 mm, whereas forone according to the present invention was used a so-called elementhaving the wedge-shaped forward end in which thinning was performedusing a position apart from the forward end by 25 mm as a starting pointin the above mentioned shape and size so as to make the forward end tohave a width of 5 mm and a thickness of 2 mm.

As the result thereof, as shown in FIG. 3, it has been determined thatthe oxygen sensor according to the present invention has a quicktemperature-rising time in reaching 400° C. That is, the time requiredto reach 400° C. is about half the time as compared with theconventional oxygen sensor, which provides an extreme effect.

Furthermore, the oxygen sensor of the present invention has its weightof the forward end portion of the sensor element lighter than that ofthe conventional one, so that the stress applied to the supporting andfixing portion of the sensor element due to vibration and shocks becomessmall, thereby the sensor element becomes difficult to be destroyed.

Another embodiment for the increase in the heating efficiency of thesensor element by the exothermic element and the decrease in the stressapplied to the supporting and fixing portion of the sensor element is asshown in FIG. 4. That is, a reinforcing material 6 supporting portion 1bof a thin plate-shaped sensor portion may be laminated to form a thinportion. Such construction, however, results in the increase in cost dueto the addition of the laminating step of the reinforcing material 6 ascompared with the case in which thinning is performed continuously, aswell as the following problems take place from a view of function.Namely, when a rapid change in temperature takes place at the sensorportion 1a due to rapid heating, rapid cooling or the like, then thebody portion 1b, which has a larger heat capacity than the sensorportion 1a, cannot follow the change in temperature, so that adifference in thermal expansion is generated between the sensor portion1a and the supporting portion 1b, and the stress caused thereby isconcentrated at the boundary portion thereof permitting crack breakdownto easily take place at the boundary portion. In addition, there may bea case in which problems occur with respect to reliability in such anenvironment in which the rapid heating is required during the start-up.

However, by continuously thinning the wall thickness from the forwardend of the sensor element 1, the concentration of the stress asdescribed above can be relieved, and such a further effect can beobtained that an oxygen sensor which is excellent in reliability withrespect to the rapid change in temperature can be obtained.

In addition, in order to satisfy the above mentioned characteristics, itis preferable to make the thickness of the body portion 1b of the sensorelement 1 to be 2 to 6 mm (in case of not less than 6 mm, thetemperature characteristics decrease due to increase in the heatcapacity, and in case of not more than 2 mm, the incorporation propertyis deteriorated due to decrease in the strength), and it is desirablefor the width of the body portion of the sensor element 1 to be 3 to 6mm after taking the incorporation property into the housing 2 and thearrangement of the electrodes 105 and 107 and the exothermic element 103and the like into consideration, as well as for the length of thewedge-shaped portion at the forward end of the sensor element 1 to be 10to 30 mm from a view of the characteristics of the oxygen sensor.

Incidentally, the above mentioned sensor element 1 has been squarepillar-shaped, which may be polygonal pillar-shaped, and for example,even in the case of such supports 401 of a cylinder or a ellipsoidalcylinder shape as shown in FIG. 5(a), (b), and (c), the same effect canbe obtained.

FIG. 6 and FIG. 7 show other examples of the sensor element of thepresent invention.

In FIG. 6, a reference electrode 105 and a measuring electrode 107,which are provided at a solid electrolyte layer 106, are arranged so asto cover not only an opening portion at a slant face 101a of a support101 but also the whole part of the slant face, wherein the electrodelayers 105 and 107 which are excellent in thermal conductivity arearranged at the whole part as described above, thereby the uniformheating property of the sensor element is increased, and increase indurability against heat shocks can be contemplated.

FIG. 7 is characterized by providing an exothermic element 103 and asensor portion at the same face side of a support 101, thereby theheating efficiency of the exothermic element 103 can be furtherincreased.

FIG. 8(a) and (b) show still another embodiment of the invention of thepresent application FIG. 8(a) and (b) are characterized in that one endof a body portion forms a slant face subjected to continuous thinning tobe a thin plate, and a portion at which the sensing electrode and aprotecting layer 108 are formed forms a planar plate shape. By adoptingsuch construction, there is little thermal gradient at the measuringportion, so that detection signals can be stably obtained.

FIG. 9 shows still another example of the invention of the presentapplication, the thin portion formed at one end of the body portion hasbeen provided with the slant face subjected to continuous thinning fromone face of side faces in the above mentioned example, however, in thepresent example a thinned portion is provided which is subjected tocontinuous thinning from the upper face or the lower face, and thethickness of the sensor element has been made constant. Owing to suchconstruction, the heat from the exothermic element can be uniformlyreceived by a detecting element 1a to obtain stable detection signals.FIG. 10 shows another example of the invention of the presentapplication. FIG. 10 is characterized in that throughout both side facesof an element are formed slant faces opposing one another subjected tocontinuous thinning from one end of a body portion, and at the measuringportion is provided a planar plate shape. Also in such construction,there is little thermal gradient at the measuring portion, so thatstable detection signals can be obtained.

Incidentally, the heat has been mainly obtained from the platinum metalmaterial in the above mentioned examples, however, other than the above,a heater may be available which is composed of a material of highmelting temperature such as tungsten, rhenium, molybdenum and the like,or an alloy material thereof.

Moreover, the protecting layer has been formed by means of the flamespray coating of the material powder in the above mentioned examples,however, other than the above, after lamination in a state of nocalcination, simultaneous calcination with a support may be carried out.

EFFECTS OF THE INVENTION

The present invention is constructed as described above, so that thereare provided such effects as described hereinafter.

(1) The sensor element becomes difficult to be destroyed due tovibration and shocks, so that in addition to that the reliability of theoxygen sensor increases, the temperature-rising speed of the sensorelement given by heating of the exothermic element becomes rapid,therefore, sufficient function is exhibited even in the case in whichrapid heating is required during the start-up and the like.

(2) In addition, the sensor element is made to be a square pillar-shapedconfiguration with the forward end portion of a wedge-shapedconfiguration, thereby the sensor element which is strong againstvibration and shocks and has a rapid temperature-rising speed can bemanufactured with ease.

INDUSTRIAL APPLICABILITY

As described above, the oxygen sensor according to the present inventionis used for detecting oxygen concentration in an exhaust gas dischargedfrom an internal combustion engine.

We claim:
 1. An oxygen sensor having a sensor element and an exothermicelement, said sensor element comprising:a support element forintroducing a reference gas, said support element including a bodyportion, said body portion having a reduced thickness portion, saidreduced thickness portion having a thickness which is thinner than athickness of a remainder of said body portion, said reduced thicknessportion having an opening for the introduction of the reference gas,said opening extending through said body portion; and a detectingelement formed at said reduced thickness portion for measuring adifference of oxygen concentration between a measured gas and saidreference gas passed through said opening; said exothermic element beingformed at said reduced thickness portion of said support element forheating said sensor element.
 2. The oxygen sensor according to claim 1,wherein said body portion has a rod-shaped configuration and one endthereof includes the reduced thickness portion, said reduced thicknessportion subjected to a reduction in thickness in accordance with adirection toward a forward end of said body portion.
 3. The oxygensensor according to claim 1, wherein said body portion is composed of anelement having a square pillar-shaped configuration in which a forwardend portion thereof is made to be a wedge-shaped configuration definingsaid reduced thickness portion.
 4. An oxygen sensor comprising:an oxygenconcentration detecting element for detecting oxygen concentration in anexhaust gas; and a sensor element which is formed by integrallylaminating together with an exothermic element for heating the oxygenconcentration detecting element, wherein a portion of said sensorelement being exposed to the exhaust gas includes a reduced thicknessportion, said reduced thickness portion being directed toward a forwardend of said sensor element.
 5. The oxygen sensor according to claim 4wherein said sensor element is composed of an element having a squarepillar-shaped configuration in which said forward end has a wedge-shapedconfiguration defining said reduced thickness portion.
 6. An oxygensensor comprising:a sensor element formed to be of a squarepillar-shaped configuration and having a reduced thickness portion atone end thereof, said reduced thickness portion formed to be a thinplate configuration in accordance with a direction toward a forward endof said sensor element, said reduced thickness portion having an openingfor the introduction of a reference gas, a detecting element formed atsaid reduced thickness portion of said sensor element for measuring adifference of oxygen concentration between a measured gas and saidreference gas passed through said opening, and an exothermic elementintegrally formed with said sensor element for heating said detectingelement.
 7. The oxygen sensor according to claim 6 wherein said reducedthickness portion has one side face of the square pillar-shapedconfiguration which forms a slant face so as to define said thin plateconfiguration, said thin plate configuration tapering to a forward endthereof, andsaid detecting element is formed at said slant face.
 8. Theoxygen sensor according to claim 6 wherein said reduced thicknessportion has opposing side faces which form slant faces so as to definesaid thin plate configuration, and said detecting element is formed atsaid slant face.